Infrared spectroscopy is a method of identifying substances by the infrared wavelengths they can absorb. Infrared light is on the spectrum just after visible light:
Infrared spectroscopy uses a machine, like a computer, that hits a substance, or in the case of forensic science an analyte (questioned substance), with infrared rays. The machine then records the wavelengths that the substance absorbs. The result is a graph of absorbance. The picture on the right is a collection of some absorbance graphs of different substances. The graph can then be analyzed in one of 4 ways: 1) If the substance has a suspected identity, its graph can be compared to that of a known graph. 2) A person who knows how to analyze an absorbance graph can compare the peaks in the "fingerprint region" (at frequencies between 450 and 1500 cm-1) of the graph to the accepted "literature values" regarding the specific wavelengths, shapes, and ratios of the different peaks. This works because each peak represents the absorbance of a specific compound, and each compound has unique absorbance properties. 3) The previous method can be used to analyze a graph in the section of the graph that has the principal peaks, or peaks representing the strongest absorptions. 4) Using a computer, the graph can be compared to a database of known absorbance graphs. (White, 1998) |
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IR is a convenient and effective method of identifying a substance. The machine does most of the work, and computer databases are very useful for quickly validating the suspected identity by comparing the graph to known graphs. Chemists trained in spectroscopy also know how to read a graph and determine what compounds are indicated by specific peaks. Now that infrared spectroscopy has been defined, see how it is used to solve crimes.
Infrared Spectroscopy |
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